Re: BASIC and System ROM replacements

From: Jim Brain <>
Date: Mon, 03 Jun 2013 20:32:58 -0500
Message-ID: <>
On 5/30/2013 1:55 AM, Gerrit Heitsch wrote:
> On 05/30/2013 08:22 AM, Jim Brain wrote:
>> Speaking (writing) of bank registers, anyone know of a TTL option that
>> has a falling edge trigger?  Yes, I can do an inverter, but would rather
>> not.  Use case is VIC-20, which won't work on rising edge of Phi,
>> because not all of the data is valid on rising edge.
> The 74LS373 or 74LS573 does that. But it's a latch, not a register. 
> They use it in the C64 to de-multiplex the VIC address so it can 
> access the char ROM. Trigger is the falling edge of _RAS.
> Now you have to check if a latch works in your case.

A latch (in general) would work in this case, as the outputs don't need 
to be valid until PHI2 goes high again, which would be 500nS later.

Thus, I appreciate the suggestion.  But, as I look at replacing, I 
realize I forgot the latch needs an enable line as well.  Neither of 
those offer one.

I'm trying to create a "register" at Address 38920 (on the VIC) to hold 
the FLASH ROM bank.  I have 38920 decoded as active low, and then an 
inverted PHI into the CLK line of a '173 is latching the bank.  But, I'd 
rather use a single IC solution.


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Received on 2013-06-04 02:00:03

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